This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Ultrasound transducers are used to transmit and receive ultrasonic signals in a flowmeter system. Several problems were solved by using the present invention, they are: The ability to remove or repair transducers from existing ultrasonic flow meters required either depressurizing the pipe gas line or the use of special tools for transducer replacement; transducers not in direct contact with the gas contained in the pipe usually have poor transducer performance resulting in poor signal detection for gas meters. Current ultrasonic gas meters use gas “wetted” transducers, that is, the transducer itself is in direct contact with the gas and cannot be removed without special tools or the elimination of the pressure in the pipe. Traditional transducer housings that put the transducer outside of the pipe pressures, such as those are used in liquid meters, have not been used since the acoustic losses through the housings combined with the noise produced by the mounting of the transducer housings make the acoustic signals unusable. The traditional transducer housings do not work in a gas environment for two reasons. First, conventional transducer housings had thick metal windows that poorly match the acoustic impedance of gas and therefore would not transmit sound into the gas. Second, the traditional transducer housings are rigidly attached to the meter body. These rigid attachments provide pathways for sound to be transmitted through the meter body and not through the gas resulting in poor signal to noise ratios.
The current ultrasonic gas meters have their wetted transducers exposed to the gas which can contain hydrogen sulfide or other contaminants. Hydrogen sulfide can over time deteriorate transducers made of conventional solder joints and epoxies for electrical and mechanical connections within the transducer. In current ultrasonic gas meters, a metal seal is placed behind the transducer in order to maintain the pipe pressure and to prevent gas from leaking out. So if a transducer fails, the transducer has to be replaced using special tools that prevent the transducer from bursting out of the flowmeter at high speeds due to pipe gas pressure. This may be deadly to the replacer if the tools are improperly handled because the transducer becomes a projectile under pipe gas pressure. In addition, the escaping gas is usually highly flammable, presenting an extreme hazard to the replacer and others nearby. If the gas pipeline/process is depressurized to replace transducers for safety reasons the pipeline flow ceases causing lost revenue.
Current gas meter transducers use either a monolithic PZT ceramic transducer or a Tonpilz transducer. These transducers suffer from poor bandwidth, poor signal to noise ratio and radial modes in the 100 to 300 kHz frequency range, the preferred ultrasonic operating frequency range for gas meters. As a result, the received signals can be very distorted. This results in poor transit time measurements and poor accuracy of a gas flowmeter Current gas meters also have metal to metal contact between parts even with wetted transducers when cases enclosing the transducers are made of metal so the flow meter can suffer from poor signal to noise ratio due to acoustic noise of the system.